Spectroscopic Confirmation of Four Ultra Diffuse Galaxy Candidates In

Spectroscopic Confirmation of Four Ultra Diffuse Galaxy Candidates in Group Environments Research Thesis Presented in partial fulfillment of the requirements for graduation with research distinction in Astronomy and Astrophysics in the undergraduate colleges of The Ohio State University by Conor Hayes The Ohio State University May 2020 Project Advisors: Dr. Paul Martini, Dr. Johnny Greco Abstract Ultra diffuse galaxies (UDGs) are a type of low surface brightness galaxy that have gar- nered much interest in the astronomical community in recent years in part because their high dark to baryonic matter ratios make them ideal testing grounds for the assumptions of the ΛCDM cosmological framework. Measuring redshifts of these objects is of critical importance because it allows us to understand their physical properties and, through comparisons with established galaxy catalogues, the environments in which they live. In this work, I present redshift measurements for four UDG candidates. Through these measurements, I have determined that these objects exist in group environments. Because the number of UDGs in similar environments with confirmed distances is presently quite small, this represents an important addition to our catalogues of the low surface brightness universe. Contents 1 Introduction2 2 UDG Candidate Characterization8 2.1 Sample Selection.................................8 2.2 Redshift Identification.............................. 10 2.3 Physical Properties................................ 12 3 Environments 17 4 Conclusions and Future Work 20 A Spectra 22 B Environments 27 Chapter 1 Introduction Over a period of ten days in December 1995, the Hubble Space Telescope stared at a relatively unexciting patch of the sky about 2.6 arcminutes across. The resulting image, now known as the Hubble Deep Field, revealed a universe packed full of galaxies. Despite covering only one 24-millionth of the sky, the HDF contained roughly 3,000 galaxies and became a foundational data set for astronomers studying the early universe. However, small, deep-field images like the HDF can't tell the whole story about how galaxies form. The need for a wide- field imaging survey was the primary motivation behind the project that eventually became the Sloan Digital Sky Survey (SDSS; York et al., 2000). While SDSS quickly became the most prolific extragalactic survey to date, it, like nearly all ground-based surveys, is biased towards the high surface brightness universe. This is thanks in part to the fact that the night sky, even under the most optimal of observing conditions, is not perfectly dark. Due to a multitude of factors, the sky has a minimum intrinsic surface brightness of ∼ 22 − 23 mag arcsec−2 (Leinert et al., 1998). As a consequence, any objects with a surface brightness less than this can be extremely difficult to detect from ground-based telescopes. For many decades, it was unclear if low surface brightness objects even existed. Glimmers of a potential problem emerged in Freeman(1970), where an analysis of thirty-six disk galaxies revealed that twenty-eight had average surface brightnesses in the very narrow range of 21:65±0:3 mag arcsec −1, despite the fact that their absolute magnitudes varied by nearly 2 5 mag. Seizing on the fact that this value was suspiciously close to the average sky brightness, Disney(1976) argued that low surface brightness galaxies (LSBGs) might be systematically underrepresented in galaxy population analyses. Indeed, it is now believed that LSBGs make up a large fraction of the total number of galaxies in our universe (e.g. Impey & Bothun, 1997). If we wish to accurately understand the nature of the universe through observations of galaxies, it is critically important that we accurately understand the nature of LSBGs. Due to the detection difficulties created by the sky brightness problem, data concerning LSBGs have been severely limited until quite recently. While one might think that we would be able to use space telescopes to circumvent the issues inherent to living on a planet with an atmosphere, it turns out that they aren't all that useful to low surface brightness astronomy. Firstly, they are quite expensive to launch and maintain. This has the consequence of limiting the amount of time that they can dedicate to any one project, meaning that reaching the exposure times required to obtain useful imaging data for LSBGs is quite difficult. Furthermore, they are subject to the same systematic errors as all reflecting telescopes, primarily diffraction and scattered light from the telescope assembly, which contaminate images below ∼ 29 mag arcsec −1 (Abraham & van Dokkum, 2014). This difficulty is further compounded by the fact that to obtain a statistically significant sample of LSBGs, we will need deep field surveys that can image a large portion of the sky, rather than just a tiny fraction of it. SDSS has been of some help, but even its utility to LSBG studies has been inhibited by its surface brightness limits. Fortunately, not all is lost. Recent years have seen the development and implementation of the next generation of wide-field imaging surveys. These surveys, including the Dark En- ergy Survey (DES; Dark Energy Survey Collaboration et al., 2016), the Kilo-Degree Survey (KiDS; de Jong et al., 2015), the Hyper Surprime-Cam Subaru Strategic Program (HSC-SSP Miyazaki et al., 2018), and the upcoming Vera C. Rubin Observatory (Ivezic et al., 2008), will allow us to probe the universe at ever-lower surface brightnesses. Several examples of the advantages that these surveys provide us over longer-standing surveys like SDSS are 3 presented in Figure 1.1. Despite these advances, our knowledge of the general properties of LSBGs is still severely lacking, particularly at low stellar masses. What we do know is that they are incredibly varied across all galaxy properties and that the limits of this variability have not yet been rigorously tested for a statistically significant sample (though efforts were made to do so in the 90s, e.g. Dalcanton et al., 1997a; McGaugh & Bothun, 1994; McGaugh, 1994; McGaugh et al., 1995), creating an urgent need to identify and study these objects. A subcategory of LSBGs that has generated a great deal of interest in recent years are so-called \Ultra Diffuse Galaxies", or UDGs. While the definition of what exactly constitutes a UDG is variable across the literature, they are generally characterized as being physically quite large with very small stellar masses, often spreading a dwarf galaxy's mass (∼ 107 − 9 10 M ) over an area the size of the Milky Way (reff ∼ 1:5 − 5 kpc). This results in central surface brightnesses between 25 and 27 mag arcsec −1. While the first UDG was discovered in 1984 (Sandage & Binggeli, 1984), it was not until 2015 when the Dragonfly Telephoto Array discovered 47 UDGs in the Coma Cluster (van Dokkum et al., 2015) that interest in these objects really picked up. Indeed, the discovery of these UDGs reignited the astronomical community's interest in LSBGs in general. The mechanisms behind the formation of objects like these are still an area of active research, but essentially three models have emerged: 1. UDGs live in the \high spin tail" of the galactic spin rate distribution. They formed in dark matter halos with higher than average angular momentum, resulting in their larger radii. (e.g. Dalcanton et al., 1997b; Amorisco & Loeb, 2016) 2. UDGs are the result of dwarfs with a bursty star formation history, where stellar feedback mechanisms gradually pushed the entire stellar population outward. (e.g. El-Badry et al., 2016; Di Cintio et al., 2017) 3. UDGs are \failed" L∗-scale galaxies that had their gas stripped by environmental effects after forming their first generation of stars. (e.g. van Dokkum et al., 2015) Determining which of these (if any) is the dominating process in UDG formation is a 4 complex problem. Most UDGs have been found in cluster environments, where it can be difficult to discern between features that are the result of formational processes and those that are the result of environmental interactions. As a consequence, we are interested in studying isolated UDGs, as their evolution has not been influenced by massive neighbors. In general, UDGs that have been discovered thus far have tended to follow the charac- teristic morphology-density relation seen across the galactic population in general. Those in clusters are red and quenched, while those in the field are blue and star-forming. If we can find a population of quenched UDGs outside of cluster environments, then that could suggest that cluster UDGs were not quenched by their environment, as has traditionally been believed. Instead, their physical properties would be the result of internal mechanisms inherent to their formation. Previous studies (e.g. Geha et al.(2012)) have determined that quenched dwarf galaxies 9 below a stellar mass of 10 M are extremely rare in the field, and this is consistent with the fact that UDGs discovered in the field have been blue and star-forming (Leisman et al., 2017). However, these results may be biased by the fact that these isolated UDGs were identified through their HI content, and quenched galaxies would be deficient in HI. This is part of what makes optical surveys like HSC-SSP so powerful. Because they are sensitive to the starlight of LSBGs, they can detect these objects regardless of their gas fraction (down to the survey's surface brightness sensitivity). As such, if quenched UDGs exist in the field, HSC-SSP and similar surveys should be able to find them. Studies of LSBGs, and UDGs in particular, are also important testing grounds for cosmological models. While the dominant ΛCDM model does an excellent job of predicting the large-scale structures of the universe, smaller-scale simulations have encountered a number of serious problems.

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